1. Field of the Invention
The present invention relates to a control apparatus, a control method, program and a robot to be controlled so as to determine a target position for a control subject.
2. Description of the Related Art
A “full-close control” is a type of position control for moving a control subject, such as a robot, to a target position. This type of control is performed by measuring positions of both of a drive source for driving the control subject and the control subject. The full-closed control directly measures the position of the control target, and accordingly, has an advantage of enabling a highly precise positioning control. To the contrary, the full-closed control may possibly become unstable so that a control error occurs when means for transmitting a drive force between the drive source and the control target includes error factors, for example, such as drive motor backlash, shaft slippage, elongation of a wire or bending of a link.
As countermeasures against such control error caused by the error factors, the control system may conduct position compensation and drive torque compensation. For the position compensation, for example, a method is known that calculates the position compensation relating to the target position based on results of passing, through filtering means, outputs of the respective position detectors for the drive source and the control subject. In this case, it is assumed that a relationship expression (interference matrix) that represents a positional relationship between the drive source and the control subject is constant. Here, a link mechanism including a motor and a speed reducer is given as an example. In this link mechanism, the control is based on a value of an encoder attached to the motor and is transmitted through the speed reducer. Where a reduction ratio of the speed reducer is high, there is hardly an error, and the above-described relationship expression can be uniquely expressed by the matrix and the like. However, in the case of employing such drive force transmitting tool, such as a wire, when the control subject changes a posture and a load is applied thereto, the wire is elongated, and the respective matrix elements assumed in the interference matrix are changed. Moreover, in the case of employing an interference mechanism (coupling mechanism), there is a tendency for the matrix elements to be changed more significantly, and in addition, the matrix elements will be nonlinear elements. Accordingly, it is difficult to determine the positional relationship (the relational expression) by using an accurate model. It is difficult to eliminate nonlinear variations of the positional relationship with the filtering means.
When performing such position compensation as described above by using a target position deviation, an integral element is introduced into a position control system, whereby a stationary deviation is eliminated, thus making it possible to improve control performance. However, in general, the integral element causes a phase delay, which causes control instability.
As for drive torque compensation, a control system has been developed in which tension applied to the wire is directly measured by a tension differential type torque sensor, and a feedback control is performed. However, the tension differential type torque sensor directly contacts the drive force transmitting tool, and accordingly, rigidity of the drive force transmitting tool itself is decreased. As described above, it is difficult to stably perform the target position following control.